Palm Beach Kennel Club and Sarasota Kennel Club canceled races through the weekend. Palm Beach will decide whether to resume racing Wednesday. Sarasota will race Monday.

The infection, which is not transmissible to humans, is treatable. Derby Lane set up a 24-hour infirmary so animals can be treated with antibiotics on site. Many dogs spend the six-month racing season at the track kennels. The infection thrives when dogs are confined in close quarters. The disease is not normally fatal and usually runs its course in about three days.

The racetrack, which shut down for medical reasons Monday for the second time in its 78-year history, is one of the largest tax-paying parimutuels in the state.

Vera Filipelli, the track's public relations director, said the closing will mean a loss of about $250,000 in weekly state tax revenues, plus $100,000 in weekly purses split among the 23 kennel operators.

Derby Lane will remain open for simulcast wagering amd poker in the card room. The Derby Club restaurant is closed until further notice.

Derby Lane is the only Florida track where dogs have died. Dr. Leon Sellers, the track veterinarian, said he is trying to pinpoint the cause of death.

"It's something that came around awfully quick," he said.

A greyhound that belonged to the Mendheim kennel became ill Thursday and was taken to Gateway Animal Hospital on Fourth Street N, where it died shortly after. Sellers said the dog most likely suffered from septicemia, a form of blood poisoning.

The other death, which occurred Friday, was more puzzling. "(The dog) didn't really show signs of being sick," Sellers said of the greyhound belonging System kennel.

Cultures taken from healthy and sick greyhounds at Derby Lane will be shipped to the University of Florida College of Veterinary Medicine for tests. The remains of the two dead dogs also were sent there.

More than 300 greyhounds from 2 to 3 years old -- about one-fifth of the dogs running at Derby Lane this season -- show symptoms of kennel cough, Sellers said.

"We're kind of where we've been all week with the cough," Sellers said. "Some are getting better and others coming down with it."

The outbreak is unrelated to the one in 1999, when six greyhounds died at Derby Lanefrom streptococcal toxic shock syndrome, apneumonialike illness that spread to tracks across the country. That deadly virus prompted the track to close for 15 days, its longest shutdown since World War II.

The Greyhound Protection League, a San Francisco antiracing group, has urged owners to impose a statewide quarantine at the greyhound tracks.

"We've seen what's happened before," GPL director Susan Netboy said. "There needs to be some sort of shutdown upon the first detection of illness."